HOMEOSTASIS
MAINTAINING AN INTERNAL BALANCE
Homeostasis is a process in which the body maintains an internal balance, within fluctuating limits, regardless of the changes in the external environment. It is also termed a dynamic equilibrium because the body responds to the changes in external environment by changing its internal conditions within an acceptable range to sustain life.
Some examples of homeostatic control systems are: regulation of blood sugar, of blood pH, body temperature, and oxygen.
All homeostatic control systems have three main components:
A monitor (sensor)
 located in the organs of the body, sends a signal to a coordinating centre once an organ begins to operate outside of its normal limits.
A coordinating centre
 relays the information to the appropriate regulator
A regulator
 helps to restore the normal balance by slightly altering a condition in the body to respond to the external environment.
EXAMPLE:

Exercise increases CO2 levels in blood

Chemical receptors in the brain stem are stimulated

Nerve cells in the brain carry impulses to muscles and increase breathing rate and depth

Increased breathing rate helps to flush extra CO2 in blood

Homeostasis regulatory mechanisms are of two types:
NEGATIVE FEEDBACK
Most homeostatic regulatory mechanisms use the negative feedback, which is a process by which conditions in the body are restored to original state. Regulation of blood glucose, pH and thermoregulation are examples of negative feedback mechanisms.

POSITIVE FEEDBACK
This feedback is not common; it is a process in which the controlled variable is moved further away from its steady state.
Blood clotting is an example of positive feedback mechanism. Chemicals are released which cause more and more clotting until the clot is complete.

THERMOREGULATION is the maintenance of body temperature within a range that enables cells to function efficiently. The Hypothalamus in the brain is the coordinating centre for thermoregulation, which functions to turn the “on” and “off” signals for cooling or heating systems in body.
A decrease in the environmental temperature causes the body to respond by erecting the body hair, shivering, and constricting blood vessels in skin.
An increase in environmental temperature causes the body to respond by sweating and dilation of blood vessels.

THE IMPORTANCE OF EXCRETING WASTES
To maintain life’s processes, the body must eliminate waste products. Urea, ammonia, and uric acid are some examples of waste products that the body must rid to stay at a constant state. The kidneys function by deamination of amino acids to remove these wastes from the blood.
THE URINARY SYSTEM
Wastes are filtered from the blood by the kidneys and conducted to the bladder for excretion by the ureters.
A cross-section of a human kidney:

Ureters
 tubes that conduct urine from the kidneys to the bladder
Urethra
 tube that carries urine from the bladder to the exterior of the body
Cortex
 outer layer of the kidney
Medulla
 area inside the cortex
Renal pelvis
 area where the kidney joins the ureter
Nephron  functional units of kidneys
Afferent arterioles
 small branches that carry blood to the glomerulus
Glomerulus
 high-pressure capillary bed, site of filtration
Efferent Arterioles
 small branches that carry blood away from the glomerulus
Peritubular Capillaries
 small blood vessels surrounding the nephron
Bowman’s Capsule  funnel-like part of nephron surrounding the glomerulus
Proximal Tubule
 joins the Bowman’s Capsule with the loop of Henle
Loop of Henle
 carries filterate from proximal tubule to the distal tubule
Distal Tubule

conducts urine from the loop of Henle to the collecting duct
Collecting Duct
 tube carrying urine from nephrons to the pelvis of kidneys

FORMATION OF URINE
Urine is formed by three functions:
Filtration: movement of fluids from the blood through a selectively permeable membrane into the Bowman’s capsule.
Reabsorption: transfer of essential solutes and water from the nephron back to the blood by active and passive transport.
Secretion: movement of materials such as ammonia, from the blood back into the distal tubule of the nephron.
WATER BALANCE
Regulating ADH

The increased water intake results in an increased urine output, as an adjustment in the body’s water balance.
ADH (Antidiuretic hormone) helps to regulate the osmotic pressure by causing the kidneys to increase water reabsorption. Osmoreceptors are specialized nerve cells in the hypothalamus that detect changes in the osmotic pressure.
A decrease in water intake or increase in water loss results in an increase in osmotic pressure because of the more concentrated blood solutes. More water, then, moves into the bloodstream, causing the hypothalamus to shrink. This results the pituitary gland to release ADH, which enters the kidneys through the bloodstream and reabsorbs more water. The kidneys produce more concentrated urine, and prevent the osmotic pressure to increase any further.
Kidneys and Blood Pressure
The kidney also functions to regulate blood pressure by maintaining for blood volume.
Aldestrone is a hormone in the adrenal gland that increases sodium ion reabsorption from the distal tubule to collecting duct. As salt (NaCl) increases in the blood, reabsorption increases, the osmotic pressure increases as a result and water moves out of nephron as a result. pH Balance
This is also regulated by the kidneys. An acid-base balance is maintained by buffer systems to absorb excess hydrogen ions or hydroxide ions. A bicarbonate-carbon dioxide buffer system maintains the pH balance.
KIDNEY DISEASE
Kidney disease and disorder can be detected by Urinalysis. Diabetes Mellitus and
Insipidus, Bright’s disease and kidney stones are all disorders of kidneys that affect the body’s attempts to maintain a dynamic equilibrium. Some common treatments are kidney transplants and Dialysis.

CHEMICAL SIGNALS MAINTAIN HOMEOSTASIS
IMPORTANCE OF ENDOCRINE SYSTEM
The Endocrine system involves chemical communication systems that provide the means to control a huge number of physiologic processes.
Hormones are chemicals released by cells that affect other cells in the body and
Endocrine hormones are chemicals secreted by the endocrine glands directly into the blood. Insulin is a hormone that regulates blood sugar by increasing the permeability of cells to glucose and is produced by the islets of Langerhans in the pancreas.
The Hypothalamus regulates the pituitary gland through nerve stimulation.
Hormones do not affect all cells because cells may have receptor sites for only some types of hormones. There are two types of hormones:
Steroid Hormones: group of hormones, made from cholesterol, soluble in fat and insoluble in water.
The steroid hormone molecule passes into the cell, combines with the receptor molecule and activates a gene in nucleus.
Protein Hormones: group of hormones, composed of amino acids, soluble in water and include insulin hormone.
The protein hormone combines with specific receptor sites and triggers the formation of cyclic AMP as a secondary messenger and activates enzymes within the cell.
Pituitary Gland: functions together with the hypothalamus to coordinate endocrine and nervous system
HORMONES THAT AFFECT BLOOD SUGAR
The Pancreas functions to regulate blood sugar. It is composed of Alpha Cells (produce insulin) and Beta Cells (Produce glucagon). Insulin, produced in Beta cells of the islets of Langerhans is released when blood sugar level increases. Glucagon, produced in the
Alpha cells of the islets of Langerhans is released blood sugar level decreases.

HORMONES THAT AFFECT METABOLISM
Thyroid Gland: a two-lobed gland at the base of the neck that regulates metabolic processes.
Parathyroid glands: four tiny glands in the thyroid gland that produce parathyroid hormone to regulate blood calcium and phosphate levels.

Two thyroid hormones are T3 and T4 (Thyroxine), of which T4 is mostly secreted.
NEGATIVE FEEDBACK OCCURS IN THIS REGULATION:
Decreased metabolic rate

receptors in hypothalamus activated

nerve cells secrete thyroid-releasing hormone

stimulates pituitary to release thyroid-stimulating hormone  TSH carried to thyroid gland  thyroxine released  raises metabolism by increasing sugar utilization in body
 normal rate of metabolism achieved

pathway turned “off”.
REPRODUCTIVE HORMONES:
Testosterone: male sex hormone, causes male sexual characteristics at puberty
Estrogen: inhibits facial hair growth
Progesterone: inhibits ovulation
HOW NERVE SIGNALS MAINTAIN HOMEOSTASIS
IMPORTANCE OF NERVOUS SYSTEM

The nervous system consists of two parts:
Central Nervous System(CNS) - Nerves of the brain and spinal cord
Peripheral Nervous System(PNS) -Nerves of the body which travel to the CNS.
NERVE CELL
Parts of a nerve cell:
Dendrites

projections of cytoplasm that carry impulses towards the cell
Axon

extension of cytoplasm that carries nerve impulses away from the cell body
Myelin Sheath
 insulated covering over the axon of the nerve cell
Nodes of Ranvier

regularly occurring gaps between myelin sheath along the axon
Schwann Cells

produce myelin sheath; types of glial cells
There are two different types of nerve cells.
GLIAL CELLS

non-conducting cells that are important for structural support and metabolism of nerve cells
NEURONS
 conduct nerve impulses
SENSORY NEURONS carry impulses from sensory receptors to the CNS.
MOTOR NEURONS carry impulses from CNS to effectors.
A reflex action is the simplest type of behaviour produced by an organism in response to a stimulus. It is a rapid, automatic, inborn response that does not require any conscious thought.

ELECTROCHEMICAL IMPULSE
A resting potential is the voltage difference across a nerve cell membrane during the resting stage and as the nerve become excited, the action potential is reached, which is the voltage difference across a nerve cell membrane when a nerve is excited. The electrochemical event is caused by unequal concentrations of positive ions across the nerve cell membrane. When these positive potassium ions diffuse out of the membrane, the highly concentrated sodium ions diffuse in the cell. Consequently, more potassium ions diffuse out of the nerve cell than sodium ions move in.
SYNAPTIC TRANSMISSION:
Synapses are small spaces between neurons; neurotransmitters at the end of axons are released from vesicles into the synapses. These neurotransmitters allow the nerve message to move across the synapses.
THE CENTRAL NERVOUS SYSTEM

Cerebrum
 stores sensory information, initiates motor activities, higher order consciousness
Cerebral Cortex
 outer lining of cerebral hemispheres
Cerebellum
 muscle movement coordination
Medulla Oblongata
 autonomic nerve control
AUTONOMIC NERVOUS SYSTEM:
The autonomic nervous system is a motor system. The sympathetic nervous system prepares the body for stress, while the parasympathetic nervous system returns the body to a resting state. Acetylcholine is a neurotransmitter released from parasympathetic system and sympathetic system. Endorphine and enkephalins are pain killers released by the body naturally.
MAINTENANCE OF BALANCE BY THE IMMUNE SYSTEM
The body’s first line of defense is its skin and mucous which serve as physical barriers to prevent most infectious organisms from entering the body. The second line of defense is white blood cells (leucocytes), that fight infections in many ways. Phagocytosis is the process by which a white blood cell engulfs and destroys a microbe.

THE IMMUNE RESPONSE
The complement proteins help defend against invading microbes by tagging the microbe for phagocytosis.
Lymphocytes

white blood cells that produce antibodies
Antibodies  proteins formed within the blood that inactivate or destroy antigens
Antigens
 stimulate the formation of antibodies
T cell
 lymphocyte, attacks foreign substances
B cell
 lymphocyte; produces antibodies
The sugar-protein complexes located on the cell membrane, act as markers. T cells distinguish the markers on body cells with those of invader cells.
The Receptor site is located along the cell membrane and determines which nutrient can fit.
The antibody prevents harmful toxins to attach to the receptor sites.
How the Body recognizes the harmful antigens?
-Bacterium enter body

-macrophage engulfs the bacterium and pushes antigen markers to outer membrane of macrophage.
-Helper T cells (cells with receptors that bind to fragments of antigens) identify the antigen on cell membrane of macrophage.
-The B cells identify the blueprint of antigen marker and begin to produce antibodies.
-Antibodies attach to the antigens.
A memory B cell is a cell that remembers the information about the geometry of an antigen.
The immune system can have malfunctions with autoimmune disease or immune-deficiency disease. Allergies occur when the immune system mistakes the harmless cells for harmful invaders. When the lymphocytes treat the body cells as aliens, immune-deficiency disease occurs.
Diseases can be transmitted by host to host, droplet infection, water, direct contact or vectors such as an infectious insect. Not all microbes cause disease and not all diseases are caused by microbes. Viruses are much smaller than bacteria, and are not cells like bacteria. The immune system cannot defeat the virulent diseases such as rabies.